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download Audio drivers for your Sound card from official website of your sound card. Search drivers on website using model number of your card. restart pc when installed driver. Should work if the problem you mentioned is as simple

Here's a link to the Emachine support page for your model. Scroll down to audio section and you should be able to download your sound drivers. If the sound still doesn't work after doing this download the chipset drivers and then try downloading the audio drivers again. Sometimes you need the chipset drivers first before the audio drivers will work.
http://www.emachines.com/support/product_support.html?cat=Desktops&subcat=T%20Series&model=T5048

check the series, is it really require realtek? because the driver doesn't depends on the windows XP, but the type, model and series of motherboard or chipset brand....
anyway, realtek doesnt written on the device manager,
check to your control panel - sound and audio device, is all enabled?or does any volume elements are muted..

after re-installl the XP u should install the drivers for audio ,video,etc. those drivers are in motherboard cd,it is provoided to u when u buying the PC.check for the motherboard cd for your computer or try to know ur motherboard model and serch through internet.